From e280581719af9ac5078eabd5641569f058fd71e3 Mon Sep 17 00:00:00 2001
From: Hiro Funatsuka <hiro.funatsuka@gmail.com>
Date: Wed, 29 Apr 2026 16:26:25 +0900
Subject: [PATCH] Solve best time to buy and sell stock

---
 .../problem.md                                |  1 +
 121_best_time_to_buy_and_sell_stock/step1.md  | 26 +++++++++++++++++++
 121_best_time_to_buy_and_sell_stock/step2.md  | 20 ++++++++++++++
 121_best_time_to_buy_and_sell_stock/step3.md  | 20 ++++++++++++++
 4 files changed, 67 insertions(+)
 create mode 100644 121_best_time_to_buy_and_sell_stock/problem.md
 create mode 100644 121_best_time_to_buy_and_sell_stock/step1.md
 create mode 100644 121_best_time_to_buy_and_sell_stock/step2.md
 create mode 100644 121_best_time_to_buy_and_sell_stock/step3.md

diff --git a/121_best_time_to_buy_and_sell_stock/problem.md b/121_best_time_to_buy_and_sell_stock/problem.md
new file mode 100644
index 0000000..18c959d
--- /dev/null
+++ b/121_best_time_to_buy_and_sell_stock/problem.md
@@ -0,0 +1 @@
+## 問題: [121. Best Time to Buy and Sell Stock](https://leetcode.com/problems/best-time-to-buy-and-sell-stock/description/)
diff --git a/121_best_time_to_buy_and_sell_stock/step1.md b/121_best_time_to_buy_and_sell_stock/step1.md
new file mode 100644
index 0000000..7d89f51
--- /dev/null
+++ b/121_best_time_to_buy_and_sell_stock/step1.md
@@ -0,0 +1,26 @@
+# Step 1
+
+- 変数を以下の通りにする
+  - stockを買う日: `buy_day`
+  - その日の値段: `price[buy_day]`
+  - 最大利益: `max_profit`
+- forループを回していく中で、`price[i]`が`price[buy_day]`より安い場合は`price[buy_day]`を`price[i]`に更新する。高い場合は`max_profit`を更新する
+
+```python
+class Solution:
+    def maxProfit(self, prices: List[int]) -> int:
+        max_profit = 0
+        buy_day = 0
+
+        for i, price in enumerate(prices):
+            if price < prices[buy_day]:
+                buy_day = i
+            else:
+                max_profit = max(max_profit, price - prices[buy_day])
+
+        return max_profit
+```
+
+時間計算量: O(n)
+
+空間計算量: O(1)
diff --git a/121_best_time_to_buy_and_sell_stock/step2.md b/121_best_time_to_buy_and_sell_stock/step2.md
new file mode 100644
index 0000000..c152f0d
--- /dev/null
+++ b/121_best_time_to_buy_and_sell_stock/step2.md
@@ -0,0 +1,20 @@
+# Step 2
+
+- ループ中にインデックスを気にせずとも書けた
+
+```python
+class Solution:
+    def maxProfit(self, prices: List[int]) -> int:
+        max_profit = 0
+        buying_price = prices[0]
+
+        for price in prices:
+            buying_price = min(buying_price, price)
+            max_profit = max(max_profit, price - buying_price)
+
+        return max_profit
+```
+
+時間計算量: O(n)
+
+空間計算量: O(1)
diff --git a/121_best_time_to_buy_and_sell_stock/step3.md b/121_best_time_to_buy_and_sell_stock/step3.md
new file mode 100644
index 0000000..aa264fd
--- /dev/null
+++ b/121_best_time_to_buy_and_sell_stock/step3.md
@@ -0,0 +1,20 @@
+# Step 3
+
+```python
+class Solution:
+    def maxProfit(self, prices: List[int]) -> int:
+        max_profit = 0
+        buying_price = prices[0]
+
+        for price in prices:
+            buying_price = min(buying_price, price)
+            max_profit = max(max_profit, price - buying_price)
+
+        return max_profit
+```
+
+1回目: 1分　9秒
+
+2回目: 1分　10秒
+
+3回目: 1分　11秒